Critical Current Mode Buck LED Driver IC LC5910S Data Sheet

Critical Current Mode Buck LED Driver IC LC5910S Data Sheet

LC5910S -DSE Rev.2.2 SANKEN ELECTRIC CO., LTD 1 Mar.01, 2017 http://www.sanken-ele.co.jp © SANKEN ELECTRIC CO., LTD. 2016

Description LC5910S is the buck single-output LED driver IC that

achieves the LED drive with few components, high efficiency, and high accuracy. The IC has various protections and configures various LED circuits. The PWM dimming and the three-level dimming (the reference voltage selection function for LED current limiting) are available.

Features Converter Parts

● Critical Current Mode (CRM) Control (Low switching loss, low noise, reduction in the number of components)

● Enable Function ● Bottom Detection Voltage 100 mV (typ.)

LED Control Parts

● PWM Dimming Function ● Reference Voltage Selection Function for LED

Current Limiting (three levels, 75% to 100%) ● Current Reference Accuracy: ±1%

Protections

● LED Output Short Circuit Protection: Pulse-by-pulse ● Current Detection Short Circuit Protection: Auto-

restart ● Input Voltage Drop Protection: Auto-restart ● Overvoltage Protection (OVP): Auto-restart, fault

output ● Thermal Shutdown (TSD) with Hysteresis: Auto-

restart

Typical Application

1 42

CS

3

8 57 6

VC

CG

ND

OU

T

SEL

PWM

FAU

LTB

D

RBD1 CBD

COUTDS

VIN

CIN

CVCC

RBD2

Q1

RCS

L1

LED

Fault SignalPWM Signal

LED Current Switch Signal

VCC

Package SOP8

Not to scale

Specification ● Recommended Supply Voltage: 12 V (min.) to 17 V

(max.) ● Circuit Current at Standby: 100 μA (typ.)

Application ● LED Back Light ● LED Lighting Equipment ● LED Bulbs

http://www.sanken-ele.co.jp/

LC5910S


Contents

Description ------------------------------------------------------------------------------------------------------ 1 Contents --------------------------------------------------------------------------------------------------------- 2 1. Absolute Maximum Ratings ----------------------------------------------------------------------------- 3 2. Recommended Operating Range ----------------------------------------------------------------------- 3 3. Electrical Characteristics -------------------------------------------------------------------------------- 4 4. Block Diagram --------------------------------------------------------------------------------------------- 5 5. Pin Configuration Definitions --------------------------------------------------------------------------- 5 6. Typical Applications -------------------------------------------------------------------------------------- 6 7. Physical Dimension --------------------------------------------------------------------------------------- 7 8. Marking Diagram ----------------------------------------------------------------------------------------- 8 9. Operational Description --------------------------------------------------------------------------------- 9

9.1 Operation in Critical Current Mode (CRM)--------------------------------------------------- 9 9.2 Enable ------------------------------------------------------------------------------------------------- 9 9.3 Reference Voltage Select Function for LED Current Limit -------------------------------- 9 9.4 LED Current Setting ----------------------------------------------------------------------------- 10 9.5 Bottom Detection Function (Off-time Period Termination Detection)------------------ 10 9.6 Maximum On-time -------------------------------------------------------------------------------- 10 9.7 PWM Dimming ------------------------------------------------------------------------------------ 11 9.8 External Power MOSFET Gate Drive -------------------------------------------------------- 11 9.9 Overvoltage Protection (OVP) ------------------------------------------------------------------ 12 9.10 Fault Signal ----------------------------------------------------------------------------------------- 12 9.11 Thermal Shutdown (TSD) ----------------------------------------------------------------------- 12

10. Circuit Constant Setting ------------------------------------------------------------------------------- 13 10.1 Inductor Setting ----------------------------------------------------------------------------------- 13 10.2 Input Smoothing Electrolytic Capacitor, CIN, Setting -------------------------------------- 15 10.3 Output Smoothing Capacitor, COUT, Setting ------------------------------------------------- 15 10.4 Current Detection Resistor, RCS, Setting ----------------------------------------------------- 16

11. Design Notes ---------------------------------------------------------------------------------------------- 17 11.1 External Components ---------------------------------------------------------------------------- 17

11.1.1 Inductor --------------------------------------------------------------------------------------- 17 11.1.2 Input and Output Smoothing Electrolytic Capacitor -------------------------------- 17 11.1.3 Current Detection Resistor ---------------------------------------------------------------- 17 11.1.4 Freewheeling Diode ------------------------------------------------------------------------- 17 11.1.5 External Power MOSFET ----------------------------------------------------------------- 17

11.2 PCB Trace Layout -------------------------------------------------------------------------------- 18 12. PCB Pattern Layout ------------------------------------------------------------------------------------ 19 13. Reference Design of Power Supply ------------------------------------------------------------------ 20 14. Design Flow Chart -------------------------------------------------------------------------------------- 21 Important Notes ---------------------------------------------------------------------------------------------- 22


LC5910S


1. Absolute Maximum Ratings Current polarities are defined as follows: current going into the IC (sinking) is positive current (+); and current

coming out of the IC (sourcing) is negative current (−). Unless specifically noted, TA = 25 °C.

Parameter Symbol Pins Rating Unit

CS Pin Voltage VCS 1−3 −0.3 to 18.0 V

VCC Pin Voltage VCC 2−3 −0.3 to 18.0 V

OUT Pin Voltage VOUT 4−3 −0.3 to 18.0 V

BD Pin Voltage VBD 5−3 −1.0 to 18.0 V

FAULT Pin Voltage VFAULT 6−3 −0.3 to 18.0 V

PWM Pin Voltage VPWM 7−3 −0.3 to 18.0 V

SEL Pin Voltage VSEL 8−3 −0.3 to 18.0 V

OUT Pin Sink/Source Current IOUT 4−3 −1.5 to 1.5 A

BD Pin Sink/Source Current IBD 5−3 −4.0 to 4.0 mA

Allowable Dissipation(1)(2) PD ― 1.2 W Thermal Resistance between Junction

and Lead (pin 3) θj- Pin ― 65 °C/W

Thermal Resistance between Junction and Ambient (2) θj-A ― 95 °C/W

Junction Temperature(3) Tj ― 150 °C

Operating Ambient Temperature(1) Top ― −40 to 125 °C

Storage Temperature Tstg ― −40 to 150 °C (1) Limited by junction temperature. (2) The IC is mounted on the glass-epoxy board (40 × 40 mm) with copper area (25 × 25 mm). (3) The temperature of thermal shutdown operation is 150 °C (typ.).

2. Recommended Operating Range The recommended operating range shows the operating conditions that are required for maintaining the normal

circuit function shown in the electrical characteristics. The IC should be used within the recommended operating range. Unless specifically noted, TA = 25 °C.

Parameter Symbol Min. Typ. Max. Unit

Input Voltage Range VCC 12 ― 17 V Operating Ambient Temperature Range TOP −40 ― 85 °C


LC5910S


3. Electrical Characteristics Current polarities are defined as follows: current going into the IC (sinking) is positive current (+); and current

coming out of the IC (sourcing) is negative current (−). Unless specifically noted, TA = 25 °C, VCC = 12 V.

Parameter Symbol Conditions Pins Min. Typ. Max. Unit

Operation Start Voltage VCC(ON) 2−3 10.0 11.0 12.0 V

Operation Stop Voltage VCC(OFF) 2−3 9.0 10.0 11.0 V

Operation Start Stop Hysteresis VCC(HYS) 2−3 0.5 1.0 3.0 V

Circuit Current in Operation ICC(ON) 2−3 ― 1.1 1.8 mA

Circuit Current in Non-operation ICC(OFF) VPWM = 0 V 2−3 ― 100 170 μA

PWM Pin On Threshold Voltage VPWM(ON) 7−3 1.6 2.0 2.4 V

PWM Pin Off Threshold Voltage VPWM(OFF) 7−3 0.8 1.1 1.4 V

PWM Pin Sink Current IPWM VPWM = 3.3 V 7−3 0.50 1.25 3.00 μA

Standby Mode Switching Time tST_BY VPWM = 3.3 V → 0 V 7−3 20 36 57 ms

PWM Disable VSEL Pin Voltage VSEL_DIS VPWM = 3.3 V 8−3 0 ― 0.4 V

CS Pin Reference Voltage 1 VCS1 1−3 742.5 750 757.5 mV

CS Pin Reference Voltage 2 VCS2 1−3 990 1000 1010 mV

CS Pin Reference Voltage 3 VCS3 1−3 1089 1100 1111 mV CS Pin Overvoltage Protection Threshold Voltage 1 VCSOVP1 VPWM = 3.3 V 1−3 2.4 2.7 3.1 V

CS Pin Overvoltage Protection Threshold Voltage 2 VCSOVP2 VPWM = 0 V 1−3 0.40 0.72 1.00 V

CS Pin Blanking Time tLEB 1−3 320 ns

Overvoltage Protection Restart Time tRESTART 1−3 5.0 11.0 19.0 ms

Maximum On-time tONMAX VCS = 0 V 4−3 15.0 20.0 29.0 μs

BD Pin Detection Voltage VBD(L) 5−3 70 100 170 mV

BD Pin Detection Hysteresis Voltage VBD(HYS) 5−3 ― 100 ― mV

BD Pin Time Out Time 1 tBD(TOUT1) VCS = 1.5 V 5−3 15.0 20.0 29.0 μs

BD Pin Time Out Time 2 tBD(TOUT2) VCS = 0 V 5−3 300 570 900 μs

BD Pin Detection Mask Time tBDMSK 5−3 0.3 0.62 1.10 μs

OUT Pin On-resistance (High)* ROUT(HIGH) IOUT = 30 mA 4−3 ― 30 ― Ω

OUT Pin On-resistance (Low)* ROUT(LOW) IOUT = −30 mA 4−3 ― 15 ― Ω

FAULT Pin Output Resistance RFAULT 6−3 300 610 1000 Ω Thermal Shutdown Operation Temperature* TSD — ― 150 ― °C

Thermal Shutdown Temperature Hysteresis* TSD(HYS) — ― 33 ― °C

* Ensured by design, not product tested.


LC5910S


4. Block Diagram

PWM

VCC

SEL

CS

OUT

GND

BD

FAULT

UVLO11 V / 10 V

Reg.

UVLO

WDTPWMo = L → 36 ms

Standby

Reg.

EN

TSD150 °C / 117 °C

Maximum On-time20 µs

1.25 µA

1.25 µA

CS Reference SelectorVSEL ≤ 0.4 V, Disable0.75V ≤ VSEL ≤ 1.25 V, VCS1 = 0.75 V 1.75V ≤ VSEL ≤ 2.25 V, VCS2 = 1.00 V 2.75V ≤ VSEL ≤ 3.25 V, VCS3 = 1.10 V

VCS

2.7 V

0.72 V

LEB

R

S

Q

100 mV / 200 mV

BD Time-out20 µs / 570 µs

BD BlankingOUT H→L

GATE

tON_MAX

VCC

2

1

6

4

3

5

PWMo

PWMo

CS_OVP

8

7

Figure 4-1. LC5910S Block Diagram

5. Pin Configuration Definitions

1

4

2

CS

3

8

5

7

6

VCC

GND

OUT

SEL

PWM

FAULT

BD

Pin Name Descriptions

1 CS Current Detection Signal Input.

2 VCC

Power supply voltage input (within the recommended operating range, 12 V to 17 V). A capacitor is connected between the VCC and GND pins in order to supply stable power supply voltage to the IC.

3 GND Ground

4 OUT Signal output for the gate drive of the power MOSFET.

5 BD The resistor for adjusting the off-time is connected.

6 FAULT

Fault signal output. This pin outputs the signal (the signal can be used for the setting to stop the pre converters such as PFC.)

7 PWM PWM dimming signal input. This pin inputs the PWM signal for dimming the LED string.

8 SEL

Reference voltage select signal input. This pin inputs the DC voltage signal to select the internal reference voltage that controls the LED current.


LC5910S


6. Typical Applications

1 42

CS

3

8 57 6

VC

CG

ND

OU

T

SEL

PWM

FAU

LTB

D

RBD1 CBD

COUTDS

Power Supply Input, VIN

CIN

CVCC

RBD2

Q1

RCS

L1

LED

Fault SignalPWM Signal


VCC

IL

Figure 6-1. LC5910S Typical Application


LC5910S


7. Physical Dimension

● SOP8 Package

● SOP8 Land Pattern Example

1.27

0.95

0.64

5.25

Dimensions in millimeters

NOTES:

- Dimensions in millimeters - Bare lead frame: Pb-free (RoHS compliant)


LC5910S


8. Marking Diagram

1

8

Part NumberLC5910

S K Y M D

Control Number

Lot Number: Y is the last digit of the year of manufacture (0 to 9) M is the month of the year (1 to 9, O, N or D) D is a period of days, 1: the first 10 days of the month (1st to 10th) 2: the second 10 days of the month (11th to 20th) 3: the last 10-11 days of the month (21st to 31st)


LC5910S


9. Operational Description All the characteristic values given in this section are

typical values, unless they are specified as minimum or maximum. Current polarities are defined as follows: current going into the IC (sinking) is positive current (+); and current coming out of the IC (sourcing) is negative current (−).

9.1 Operation in Critical Current Mode (CRM)

Figure 9-1 shows the LED drive circuit example using the LC5910.

1 42

CS

3

8 57 6

VC

CG

ND

OU

T

SEL

PWM

FAU

LTB

D

RBD1 CBD

COUT

DS

VIN

CIN

CVCC

RBD2

Q1

RCS

L1

LED

Fault Signal

PWM Signal


VCC

IL

ID

IS

VDS

Figure 9-1. LED Drive Circuit Example using the LC5910S

The IC uses the Critical Current Mode (CRM) control. To drive the LED string, the IC controls the on-time and off-time of the external power MOSFET as follows:

● On-time period: When the CS pin detects the peak of the external inductor current, IL, the on-time period is terminated..

● Off-time period: When the BD pin detects decrease of the voltage, VDS, between drain and source of the external power MOSFET, the off-time period is terminated.

As a result, the waveform of the inductor current, IL, becomes triangular as shown in Figure 9-2. The LED current, ILED, is half of the peak value of the inductor current, IL.

The IC controls the peak value of the inductor current, IL, and stabilizes the LED current,ILED.

See Figure 4-1 and Figure 6-1 for circuit symbols. The inductor current, IL, is converted into a voltage by

the current detection resistor, RCS, and input to the CS pin. When the voltage becomes equal to the internal reference voltage, VCS, the OUT pin output is turned off.

The off-time period is set by the VDS waveform that is

input to the BD pin (see Section 9.5). When the BD pin voltage decreases to the BD pin detection voltage, VBD(L), the off-time period is terminated and the turn-on of the next cycle is started.

As described above, the IC always turns on at the VDS of the power MOSFET decreasing in the CRM control. Therefore, the CRM control reduces the switching loss and noise more than the PWM continuous current mode (CCM) which switches hard. The CRM control requires no phase compensation and fewer components, resulting in the cost reduction in system.

The oscillation frequency of the CRM control depends on the input voltage.

VDS

ID

IS

IL

0

0

0

0

t

ILED

VBD(L)

Turn-on timming

tOFF tON

VCS

Turn-off timming

Figure 9-2. CRM Control of the LC5910S

9.2 Enable The IC has the enable function. When the SEL pin

voltage decreases 0.4 V or lower, the OUT pin signal is fixed low. A pull-down resistor is connected internally between the SEL and the GND pins.

9.3 Reference Voltage Select Function for LED Current Limit

The LED current, ILED, is controlled using the peak value of the inductor current, IL. IL is converted into a voltage by the current detection resistor, RCS, and input to the CS pin. The IC controls the voltage to be equal to the reference voltage of the internal comparator, VCS. The reference voltage of the comparator can be set to three levels according to the DC voltage applied to the SEL pin (see Table 9-1). The function enables altering the LED current with the resistance of the current detection resistor, RCS, fixed.


LC5910S


If the different frequency switching noises of the other converter on PCB superimposes on the SEL pin, the RC filter shown in Figure 9-3 is required. The RC filter must be connected close to the SEL pin. Due to the delay caused by the combination of RSEL and CSEL, it takes time for the SEL pin voltage to settle at the determined VSEL after the DC voltage is applied to the SEL pin. Be sure to confirm the actual operation and adjust the constants.

Table 9-1. The Relation between SEL Pin Voltage and Internal Reference Voltage

SEL Pin Voltage, VSEL Internal Reference Voltage, VCS (typ.)

VSEL ≤ 0.40 V (Enable state)

0.75V ≤ VSEL ≤ 1.25 V VCS1 = 750 mV

1.75V ≤ VSEL ≤ 2.25 V VCS2 = 1000 mV

2.75V ≤ VSEL ≤ 3.25 V VCS3 = 1100 mV

3

8

GND

SELRSEL

DC Voltage

CSEL

LC5910S

VSEL

Figure 9-3. RC filter of SEL Pin

9.4 LED Current Setting As shown in Figure 9-2, the LED current, ILED, is

controlled so that the peak of the inductor current, IL, at the power MOSFET turn-on is equal to the reference voltage, VCS, set by the SEL pin. When the power MOSFET turns off, the energy stored in the inductor is regenerated through the freewheeling diode, DS, and the power MOSFET turns on at the timing of the end of energy regeneration. By repeating this, the waveform of the IL becomes triangular with the current determined by the RCS and the VCS at the top, and the ILED is controlled to be half of the peak value of IL. The ILED is calculated by the Equation (1).

ILED = 0.5 ×VCSRCS

(1)

where: VCS is the reference voltage set by the SEL pin (see

Section 9.2), and

RCS is the resistance of the current detection resistor.

9.5 Bottom Detection Function (Off-time Period Termination Detection)

For the off-time period setting, the voltage between drain and source, VDS, is used. The VDS is input to the BD pin from the drain pin through a coupling capacitor and a voltage dividing resistor.

As shown in Figure 9-3, when the power MOSFET turns off, the energy stored in the inductor is regenerated through the freewheeling diode, DS. When the current flowing in the DS becomes zero, the VDS decreases due to the free oscillation by the inductor, L, and the voltage resonance of parasitic capacitance between drain and source, Cds. When the BD pin voltage decreases to the BD pin detection voltage, VBD(L) = 100 mV, or lower, the function terminates the off-time period and turn on the power MOSFET.

The BD pin detection mask time, tBDMASK = 0.62 μs (typ.), prevents misdetection due to surge and ringing waveform immediately after the power MOSFET turns off.

If the BD pin voltage maintains above VBD(L) during ≥tBD(TOUT1) of 20 μs or more after the power MOSFET turns off, the power MOSFET is forcibly turned on.

The standard constant for the BD pin is as follows: When VIN = 160 V and VLED = 130 V, CBD = 15 to 100

pF, RBD1 = 63 kΩ, and RBD2 = 2.2 kΩ. If RBD1 decreases with fixing RBD2, the turn-on delay

increases.. CBDF, which is for noise countermeasure, should be connected as needed. However, adding the CBDF increases the turn-on delay.

Since the bottom detection voltage of VDS of the BD pin is VBD(L) = 100 mV, it is strictly detected a little earlier than the bottom point. (Depending on the constant of the BD pin detection circuit, the turn-on is delayed and the bottom point of VDS may be adjusted to approximately 0 V).

3

8

GND

BD RBD1

CBDFLC5910S

CBD

Q1

RCS

100 mV / 200 mV

BD Time-out20 µs / 600 µs

BD BlankingOUT H→L

RBD2

Figure 9-4. BD Pin Connection

9.6 Maximum On-time In case the current detection resistor, RCS, is shorted

or the supply voltage, VIN, decreases, the OUT pin


LC5910S


output signal of the is kept in the logic high state; and the power MOSFET may be destroyed. To prevent this event, the OUT pin output signal is limited by the maximum on-time.

When the on-time period exceeds the maximum on-time, tONMAX = 20 μs (typ.), the OUT pin output signal becomes logic low, and outputs the logic high signal again in tBD(TOUT2) = 570 μs.

9.7 PWM Dimming The PWM pin is the input pin of the PWM dimming

signal. The PWM dimming signal that satisfies the following conditions is input to the PWM pin:

On threshold voltage, VPWM(ON) = 2 V, and Off threshold voltage, VPWM(OFF) = 1.1 V. The voltage of 2.7 V to 3.3 V is recommended for the

peak voltage of the PWM signal. The pull-down resistor is connected between the PWM and the GND pins.

When the period during which the PWM pin voltage is lower than VPWM(OFF) is 36 ms or more, the LC5910S enters standby mode. In standby mode, the current consumption of the IC is reduced. When the PWM pin voltage exceeds the VPWM(ON), the IC recovers from the standby mode.

In order to decrease the dv/dt of the PWM dimming signal, the RC filter circuit should be connected to the PWM pin as shown in Figure 9-5. However, care should be taken when signals with high frequency are input.

3

7

GND

PWMRPWM

PWM signal

CPWM

LC5910S

Figure 9-5. RC filter of PWM Pin

9.8 External Power MOSFET Gate Drive Figure 9-6 shows the peripheral circuit of the OUT

pin. The OUT pin is for driving the external power MOSFET gate. The peak output voltage of the OUT pin depends on the VCC pin voltage. When the input voltage of the VCC pin is 17 V, the output voltage of the OUT pin is also around 17 V. Therefore, it is required to choose the external power MOSFET having the gate breakdown voltage of ±20 V or ±30 V, and choose the power MOSFET such that the threshold voltage between

gate and source meets VGS(TH) < VOUT in all operating temperature range.

In a general EMI noise countermeasure, the dv/dt of the gate voltage of the power MOSFET is decreased to slow the switching speed. Note that, however, the switching loss increases. The circuit resistance of the drive output incorporated in the LC5910S is as follows:

30 Ω (typ.) for source, and 15 Ω (typ.) for sink. The resistance above can not be changed. Therefore,

the switching speed is adjusted by external components as shown in Figure 9-6 and Figure 9-7.

The components in Figure 9-6 of the gate drive circuit of the power MOSFET should be chosen as follows:

RG1 is around 100 Ω, RG2 is around 10 Ω, and DG is a Schottky diode having the same breakdown voltage as the gate breakdown voltage of the external power MOSFET.

Figure 9-7 shows a method of discharging with the transistor, QG at the power MOSFET turn-off. At turn-on, the gate of the power MOSFET is charged via RG1, DG, and RG2 from the OUT pin. At turn-off, the bias voltage of QG is the forward voltage, VF, of DG. Decreasing RG2 increases the switching speed at turn-off. In order to prevent malfunction of the IC due to the rapid dv/dt of the drain voltage of the power MOSFET, RGS is added between gate and ground of the power MOSFET as needed. RGS is around 10 kΩ to 100 kΩ.

3

4

GND

RG2

LC5910S

Q1

RCSRGS

OUTDG

RG1

1CS

Figure 9-6. OUT Pin Peripheral Circuit (1)

3

4

GND

RG2

LC5910S

Q1

RCSRGS

OUTDGRG1

1CSQG

Figure 9-7. OUT Pin Peripheral Circuit (2)


LC5910S


9.9 Overvoltage Protection (OVP) When the inductance is saturated or shorted, the

voltage between both ends of the output current resistor, RCS, increases, resulting in the increase in the CS pin voltage. When the CS pin voltage meets either of the following conditions, the overvoltage protection (OVP) is activated.

● The CS pin voltage is the overvoltage protection threshold voltage, VCSOVP1 = 2.7 V, or higher

● PWM pin voltage is in the logic low state, and the CS pin voltage is the overvoltage protection threshold voltage, VCSOVP2 = 0.72 V, or higher

When the OVP operates, the OUT pin voltage becomes logic low level, and the fault signal is output from the FAULT pin.

When the CS pin voltage decreases to be VCSOVP1, or VCSOVP2, or lower, and the overvoltage protection restart time, tRESTART = 11.0 ms, is exceeded, the fault signal output is stopped and the LC5910S restarts (see Section 9.10 for the detail of the fault signal).

Though the leading edge blanking (LEB) circuit is incorporated in the CS pin, the RC filter should be added as needed as shown in Figure 9-8 when large noises are superimposed at the voltage signal between both ends of the RCS.

The standard constant for RC filter is as follows: The RCSF is around 1 kΩ, and the CCSF is around 220

pF. If the RC time constant is large, the response delay

becomes large, and the operation may be unstable. Therefore, be sure to confirm the actual operation and adjust the constant.

3

4

GND

RG2

LC5910S

Q1

RCSRGS

OUTDG

RG1

1CSRCSF

CCSF

Figure 9-8. RC Filter of CS Pin

9.10 Fault Signal Figure 9-9 shows the peripheral circuit of the FAULT

pin. FAULT pin is connected to the source of the P-channel MOSFET in the IC. When the overvoltage state is detected (see Section 9.9), the internal P-channel MOSFET turns on, and the FAULT pin voltage

increases to the voltage of the VCC pin. In normal operation, the FAULT pin is in the high

impedance state. The fault signal can be used for the protection

operation to suppress the power supply to the LED driver by sending signals via optocoupler to the primary-side off-line converter IC or the PFC IC.

CS

FAULT

2.7 V

0.72 V

VCC

PWMo

CS_OVP 1

6

GND3

Q1

RCS

VCCLC5910S

Figure 9-9. FAULT Pin Peripheral Circuit

9.11 Thermal Shutdown (TSD) When the junction temperature of the IC reaches the

thermal shutdown operation temperature, TSD = 150 °C, or higher, the thermal shutdown (TSD) is activated and stops the oscillation. The TSD has the temperature hysteresis, TSD(HYS) = 30 °C. When the temperature of the IC decreases to be TSD − TSD(HYS), or lower, the TSD automatically restarts the normal operation.

The TSD protects the IC when the loss of the IC increases due to the instantaneous short-circuit of the OUT pin. This does not guarantee the operation including the reliability in the short-circuit state for long period or the state where the heat generation continues.


LC5910S


10. Circuit Constant Setting

10.1 Inductor Setting The on-time period depends on the input voltage, VIN,

and inductance, and is determined by the point where the IC detects the peak value of inductor current, IL(PEAK). In the control of the IC, the off-time period is determined by the point where the BD pin detects the VDS drop after the regenerative current flowing through the freewheeling diode, DS, becomes zero. Therefore, the off time, tOFF, is the value that the delay time, tONDLY, of the voltage resonant part is added to the off time, tOFF_S (see Figure 10-1). tOFF_S is determined by cycle length and on-time.

The design procedure of inductor is shown below.

VDS

IL

0

0

t

ILED

VBD(L)

tOFF tON

tOFF_StONDLY

IL(PEAK)

T

Figure 10-1. LC5910S Operational Waveform

(1) Duty D

Duty D is calculated by the following equation.

D =VLEDVIN

(2)

(2) On-time, tON

When the switching period is T, the on time, tON, is as follows:

tON = T × D (3)

(3) Off-time, tOFF_S

The off time, tOFF_S, until the regenerative current flowing through the freewheeling diode becomes zero is calculated by the following equation.

tOFF_S = T − tON (4)

(4) Inductor

The peak current of the inductor, IL(PEAK), is twice the LED average current, ILED. The value of the inductor, L, such that the peak current is IL(PEAK) is calculated by the following equation.

L = VLED × tOFF_S

IL(PEAK) (5)

where: tOFF_S is calculated by Equation (19), and VLED is the voltage across the LED string.

(5) Current Detection Resistor, RCS

The RCS value is calculated by the following equation.

RCS =VCS

IL(PEAK) (6)

(6) Delay Time, tONDLY

The delay time, tONDLY, is calculated by the following equation.

tONDLY = π × √L × Cds (7)

(7) Off-time, tOFF

The off time, tOFF, is the value that tONDLY, is added to tOFF_S calculated by (19).

tOFF = tOFF_S + tONDLY (8)

(8) Corrected Frequency, fSW’

The frequency, fSW’, considering the delay time, tONDLY, is calculated by the following equation.

fSW′ = 1

(tON + tOFF) (9)

fSW’ is later than fSW that is set as a target operating frequency by the delay time, tONDLY. In order to adjust fSW’ to the set value, the value of the inductor, L, should be decreased and the on-time should be short. Even though the on-time is adjusted, the peak inductor current, IL(PEAK), is always twice the ILED if VIN, VCS, and RCS are fixed.

As shown in Equation (7), however, when L is reduced, tONDLY also becomes short. Therefore, the waveform should be confirmed on actual operation and the constant of the BD pin should be adjusted so that a sufficient delay time can be secured (the power MOSFET turns on at the bottom of VDS).


LC5910S


Based on the calculated L value, it is necessary to select an inductor that does not magnetically saturate at the maximum current. The heat generation by the DCR of the winding should be below the manufacturer guaranteed value.

● Calculation Example The value of the inductor is calculated by the

Equation (2) to (9) using the set value in Table 10-1.

Table 10-1. Set Value of Inductance Calculation

Parameter Set Value Input Voltage, VIN 160 V LED String Voltage, VLED 130 V LED Average Current, ILED 350 mA LED Current Detection Threshold Voltage, VCS

1000 mV

Target Operating Frequency, fSW 100 kHz

Capacitance between Gate and Drain, Cds

81 pF (Cds = Coss – Crss, for Coss and Crss, refer to the data sheet of the power MOSFET)

(1) Duty, D

D =130 V160 V

= 0.8125

(2) On-time, tON

By the equation, fSW = 100 kHz, the switching frequency, T, is 10 μs.

tON = 10 µs × 0.8125 = 8.125 μs

(3) Off-time, tOFF_S

tOFF_S = 10 µs − 8.125 µs = 1.875 µs

(4) Inductor

By the equation, LED average current, ILED = 350 mA, the inductor peak current, IL(PEAK) is 700 mA.

L =130 V × 1.875 μs

700 mA= 348.2 µH

From the calculated value, 330 µH should be chosen for the inductor value.

(5) Current Detection Resistor, RCS

RCS = 1000 mV

0.7 A = 1.428 Ω

(6) Delay time, tONDLY

tONDLY = π × �330 µH × 81 pF = 0.514 µs

(7) Off-time, tOFF

tOFF = 1.875 µs + 0.514 µs = 2.389 μs

(8) Corrected Frequency, fSW’

fSW′ =1

(8.125 µs + 2.389 µs) = 95.11 kHz

fSW’ is later than the target operating frequency, 100 kHz, by tONDLY. In order to adjust fSW’ to the set value, the value of the inductor, L, should be decreased and the on-time should be short.

In the calculation example above, VCS2 = 1.0 V is assumed to be 100%.

Table 10-2. shows the design values when the SEL pin voltage is switched using the inductor of the L value (330 µH) set in the calculation example and the current detection resistor, RCS (the input voltage, VIN, is constant). In the application that the SEL pin voltage is changed, it is required to choose the inductor that does not saturate at the maximum current even in VCS3.

Table 10-2. Calculation Result of Internal Reference Voltage and Design Value when SEL Pin Voltage is

Switched

Parameter VCS1 = 750 mV

VCS2 = 1000 mV

VCS3 = 1100 mV

IL(PEAK) 525 mA 700 mA 770 mA

ILED 262 mA 350 mA 385 mA

tON 6.07 μs 8.13 μs 8.90 μs

tOFF_S 1.40 μs 1.87 μs 2.05 μs

tONDLY 0.514 μs 0.514 μs 0.514 μs

T 7.98 μs 10.51 μs 11.46 μs

fSW’ 125.25 kHz 95.11 kHz 87.23 kHz


LC5910S


10.2 Input Smoothing Electrolytic Capacitor, CIN, Setting

Since the IC provides the CRM control where the inductor current fluctuates from zero to the peak, ripple current becomes larger than in case of CCM control.

The ripple current of the input capacitor, CIN, is calculated assuming the worst condition (all the power is supplied to the LED driver circuit from CIN for a period of time). In delay time, the negative current flows through CIN as shown in Figure 10-2.

VDS

ICINR

0

0

t

VBD(L)

tOFF tON

tOFF_StONDLY

IR(PEAK)1

t1

IL

0

t2

IR(PEAK)2T

ICINR- ICINR+

IL(PEAK)

Figure 10-2. CIN Ripple Current (worst condition)

The current flowing in CIN during the period, t1, in Figure 10-2 has a triangular waveform whose peak value is IR(PEAK)2. The effective value of this current, ICINR−, is calculated by Equation (10).

ICINR− = IR(PEAK)2 × �t13T

(10)

ICIN(RIPPLE) discharging from CIN during the period, t2, in Figure 10-2 has a triangular waveform whose peak value is IR(PEAK)1. The effective value of this current, ICINR+, is calculated by Equation (11).

ICINR+ = IR(PEAK)1 × �t23T

(11)

From Equation (10) and (11), the total ripple current of CIN, ICINR, is calculated by the following equation.

ICINR = �ICINR−2 + ICINR+2 (12)

The calculated value is the maximum ripple current

considering the worst condition. A capacitor that the ripple current calculated by Equation (12) can flow should be chosen. Care should be taken in using an electrolytic capacitor.The ripple current eventually should be confirmed on actual operation.

It is required to select input capacitors having sufficient margins to the maximum applied voltage according to recommended use conditions of capacitor manufacturers.

● Calculation Example The total ripple current, ICINR, of CIN, is calculated by

Equation (10), (11), and (12). If t1 = 0.7 μs, IR(PEAK)2 = 0.26 A, and the cycle, T = 10

μs (the frequency fSW = 100 kHz), ICINR−, is calculated as follows:

ICINR− = 0.26 A × �0.7 µs

3 × 10 µs= 0.039 Arms

When t2 = 6.4 μs, IR(PEAK)1 = IL(PEAK) = 0.7 A, and the cycle, T = 10 μs (frequency, fSW = 100 kHz), ICINR+ is calculated as follows:

ICINR+ = 0.7 A × �6.4 µs

3 × 10 µs= 0.323 Arms

From the above, the total ripple current, ICINR, is as follows:

ICINR = �0.0392 + 0.3232 = 0.325 Arms

From the above, a capacitor that can permit the ripple current of 0.325 Arms or higher must be selected for CIN.

10.3 Output Smoothing Capacitor, COUT, Setting

Since the IC provides the CRM control where the inductor current fluctuates from zero to the peak, it is required to connect the output smoothing capacitor, COUT, to the both ends of the LED string, and reduce the ripple voltage of the LED string.

The ripple current, ICOUTR, flowing in COUT, is calculated by Equation (13).

ICOUTR =IL(PEAK)

2√3 (13)

When the equivalent series resistor of the smoothing capacitor, COUT, is ESR, the ripple voltage, VLED(RIPPLE),


LC5910S


of COUT , is expressed by Equation (14).

VLED(RIPPLE) = IL(PEAK) × ESR (14)

A capacitor that the ripple current calculated by Equation (13) can flow is required for COUT. The voltage across COUT is the sum of VF in the LED string. It is required to select the breakdown voltage of COUT having margins to the maximum applied voltage considering the ripple voltage in Equation (14) according to recommended use conditions of capacitor manufacturers.

VDS

ICOUTR

0

0

t

VBD(L)

tOFF tON

tOFF_StONDLY

IL(PEAK)

IL

0

T

IL(PEAK)

Figure 10-3. COUT Ripple Current

● Calculation Example If IL(PEAK) = 0.7 A, the ripple current, ICOUTR, flowing

in COUT is calculated by Equation (13) as follows:

ICOUTR =0.7 A2√3

= 0.202 Arms

From the above, a capacitor that can permit the ripple current of 0.202 Arms or higher should be selected for COUT.

When ESR = 100 mΩ and IL(PEAK) = 0.7 A, the ripple voltage, VLED(RIPPLE) , of COUT is calculated by Equation (14) as follows:

VLED(RIPPLE) = 0.7 A × 100 mΩ = 70 mVp– p

It is required to select the withstand voltage of COUT having margins to the maximum applied voltage considering the ripple voltage of 70 mVp-p according to recommended use conditions of capacitor manufacturers.

10.4 Current Detection Resistor, RCS, Setting

For the current detection resistor, RCS, noninductive resistors such as metal plate resistors, metal film resistors, and carbon film resistors should be used. If inductive resistors such as winding wire type are used, malfunction may occur due to the surge voltage by parasitic inductance component.

Axial lead and radial lead resistor should be mounted with the lead as short as possible.

● RCS Loss Calculation The switching current flows in RCS in the on-time

period of the power MOSFET. The current flowing in RCS, IRCS, is calculated by the following equation.

IRCS = ILED × D (15)

where, ILED is LED current, and D is duty

The average loss of RCS, PRCS, in Equation (15), is

calculated by the following equation.

PRCS = (IRCS)2 × RCS (16)

When VCS is switched by the SEL pin input voltage, the power dissipation should be calculated with the maximum value, VCS3 = 1.1 V.

In an abnormal operation state such as a short between the drain and source pins of the external MOSFET , the detection resistor, RCS, may be damaged. In order to avoid damage, the following measures should be taken.

- Increase the RCS power rating. - Insert the protective fuse that melts before RCS is

damaged into the power supply of the LED driver circuit.

- Using the FAULT pin of the LC5910S, stop the pre-converter that generates the supply power of the LED driver circuit.


LC5910S


● Calculation Example

Table 10-3. Set Value of Current Detection Resistance Calculation

Parameter Set Value Detection Resistor, RCS 1.428 Ω

Reference Voltage, VCS 1.0 V (VSEL = 0.75 V to 1.25 V)

LED Current, ILED 0.35 A (IL(PEAK) = 0.7 A) LED String Voltage, VLED 130 V

Input Voltage, VIN 160 V Duty, D 0.8125

Using the set value in Table 10-3., the loss of

resistance is calculated by Equation (15) and Equation (16).

IRCS = 0.35 A × 0.8125 = 0.284 A

PRCS = (0.284 A)2 × 1.428 Ω = 0.115 W

11. Design Notes

11.1 External Components Components fit for the use condition should be used.

11.1.1 Inductor It is required to set design margin properly for

temperature rise due to copper loss and iron loss, and for magnetic saturation.

11.1.2 Input and Output Smoothing Electrolytic Capacitor

It is required to set a design margin properly for the ripple current, voltage, and lifetime. The electrolytic capacitor used must have high allowable ripple current for switching power supplies and low impedance.

11.1.3 Current Detection Resistor Because high-frequency switching current flows

through the current detection resistor, a resistor with low internal inductance must be chosen. In addition, the resistor is required to be selected considering the

allowable loss.

11.1.4 Freewheeling Diode The freewheeling diode, DS, that regenerates energy

during the off-time period of the external power MOSFET (see Figure 6-1) is selected from fast recovery diodes with short recovery time, trr, or from Schottky diodes.

The peak of the forward current, IF, of the freewheeling diode is twice the ILED, and flows in the tOFF period of the power MOSFET. IF flows repeatedly at the same switching frequency as the power MOSFET. Therefore, a diode should be selected considering the allowable loss.

Do not use a general rectification diode used for commercial power supply rectification because its trr is long and a large short-circuit current flows during the recovery period. This short-circuit current may cause not only the heat generation of the diode but also malfunction of the main circuit, resulting in damage.

11.1.5 External Power MOSFET

● Breakdown Voltage between Drain and Source The input voltage, VIN－VF (the forward voltage of

the freewheeling diode) is applied between the drain and source of the power MOSFET when it is off. Considering surges at turn-off, the power MOSFET whose drain-source breakdown voltage is more than twice VIN should be selected.

● Capacitance between Drain and Source (Cds) The capacitance between drain and source (Cds)

affects the delay time, tONDLY. tONDLY becomes longer as the capacitance becomes larger (see Section 10.1).

Although Cds is not described in the data sheet of the power MOSFET, it can be calculated from the output capacitance, Coss, and the feedback capacitance, Crss, as follows:

Coss = Cds − Cgd (17)

Crss = Cgd (18)

where, Cgd is the capacitance between gate and drain, From Equation (17) and (18), Cds is calculated by the

following equation.

Cds = Coss − Crss (19)


LC5910S


● Breakdown Voltage between Gate and Source The gate drive voltage of the LC5910S varies in

proportion to the VCC pin voltage. Therefore, in the specification that VCC voltage rises to the upper limit of 17 V of the recommended operating range, a power MOSFET with gate-source breakdown voltage of 20 V to 30 V should be selected.

When a stabilized voltage of 12 V is input to the VCC pin, the crest value in the output pulse of the OUT pin is about 12 V.

● Others In general, the larger the package and the lower the

on-resistance of a power MOSFET, the larger the capacitance (e.g. Ciss) between junctions. Therefore, the larger drive current is required. Considering the drive capability of the LC5910S, a power MOSFET with package size smaller than TO 220 is recommend.

11.2 PCB Trace Layout The switching power supply circuit includes high

frequency and high voltage current paths that affect the IC operation, noise interference, and power dissipation. Therefore, PCB trace layouts and component placements play an important role in circuit designing. High frequency and high voltage current loops must be as small as possible with wide trace, in order to maintain a low-impedance state (see Figure 11-1).

In addition, ground traces should be as wide and short as possible so that radiated EMI levels can be reduced.

When the input/output conditions exceed the Safety Extra Low Voltage (SELV), component layout and pattern distance considering safety standards are required.

Figure 11-1. High Frequency Loop (shaded area)

(1) Main Circuit Trace Layout The high frequency loop as shown in Figure 11-1 and the loop including the VCC pin and the GND pin are the main trace flowing switching currents.

This traces must be as wide layout and small loop as possible.

(2) Around the GND Pin In order to prevent the switching current of the main circuit from influencing the control circuit, the control ground must be separated form the main trace and be connected near the GND pin.

(3) RCS Trace Layout RCS should be placed as close as possible to the CS pin. In order to reduce noise at current detection, the detection trace must be separated, and be connected to near RCS from the CS pin.

(4) Peripheral Components of the IC The components connected to each pin must be placed as close as possible to the IC, and must be connected as short as possible. The ground of components for detection of the IC must be connected near the GND pin. Extra attention should be paid so that the IC ground does not become a common impedance with the main trace. Do not place the IC or small signal pattern just under the inductor, L. The IC may malfunction due to the leak magnetic flux.

(5) Output Smoothing Capacitor, COUT The output smoothing capacitor, COUT is placed close to the LED string with a minimum length of traces.

(6) Thermal Considerations Because the power MOSFET has a positive thermal coefficient of RDS(ON), care should be taken for thermal design. Particularly in the case of surface mount components, the drain pin pattern functions as a heat sink. Therefore, the pattern must be designed as wide as possible.

CIN DS

Q1

COUT


LC5910S


12. PCB Pattern Layout

Front Side Back Side

Figure 12-1. Pattern Layout Example

*Since this is the evaluation board, components for adjustment are included.

Figure 12-2. Pattern Layout Circuit Diagram


LC5910S


13. Reference Design of Power Supply As an example, the followings show the power supply specification and the bill of materials. For the circuit diagram

of the bill of materials, see Figure 12-2.

● Power Supply Specification Input Voltage DC250 V VCC Pin Voltage 13 V Output Voltage DC200 V LED Current 0.478 A (VSEL = 1.0V)

● Bill of Material Symbol Part Type Rating Notes

L01* Inductor 0.47 mH C1 Ceramic 1 nF C2 Ceramic 100 pF C3 Ceramic Open C4 Ceramic 15 pF, 1 kV C5 Ceramic 0.47 μF C6* Ceramic 100 pF, 50 V C7 Ceramic Open C8 Ceramic 0.47 μF, 400 V C9 Electrolytic 22 μF, 25 V C10 Electrolytic 10 μF, 250 V D01 Fast Recovery Diode 600 V, 2 A D02 Diode 60 V, 0.5 A Q1 N-channel Power MOSFET 250 V, 7.5A, 0.4Ω Q2 PNP Transistor −60 V, −0.6 A Q3 NPN Transistor 40 V, 0.6 A R0 General 1.8 kΩ R1 General Open R2 General 2.2 kΩ R3 General 33 kΩ R4 General 30 kΩ R5 General Open R6 General 470 kΩ R7 General 470 kΩ R8 General 1 kΩ R9 General 510 kΩ R10 General 10 Ω R11* General 7.5 Ω R12* General 47 Ω R13* General 10 kΩ R14* General 100 Ω R15 General 1 Ω, 2 W R16 General 2.3 Ω, 2 W R17 General Open R18 General Open R19 General 4.7 kΩ LED — — LED for Fault detection

* Adjustment is required on actual operation.


LC5910S


14. Design Flow Chart The flow chart below is specialized for selection of constants. Measures against noise and heat dissipation on actual

operation are not included. Therefore, another adjustment is required.

START

VLED = VF × n VIN ILED

Condition Setting

Target Frequency Setting

Calculation of Duty, D

tON、tOFF_S Calculation

tON ≤ 20μs

Inductance Calculation

Power MOSFET Selection

Delay Time, tONDLY

Calculation

tOFF Calculation Corrected Frequency

Calculation

VSEL Setting

RCS Setting

CIN Setting

COUT Setting

Peripheral Components Selection such as Free Wheeling

Diode

Actual Operation Confirmation

RBD, CBD Adjustment

Check the bottom-on of the BD pin wave

-form

END


LC5910S


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